Command gofmt

Without an explicit path, it processes the standard input. Given a file,
it operates on that file; given a directory, it operates on all .go files in
that directory, recursively. (Files starting with a period are ignored.)
By default, gofmt prints the reformatted sources to standard output.

Usage:

gofmt [flags] [path ...]

The flags are:

-d
Do not print reformatted sources to standard output.
If a file's formatting is different than gofmt's, print diffs
to standard output.
-e
Print all (including spurious) errors.
-l
Do not print reformatted sources to standard output.
If a file's formatting is different from gofmt's, print its name
to standard output.
-r rule
Apply the rewrite rule to the source before reformatting.
-s
Try to simplify code (after applying the rewrite rule, if any).
-w
Do not print reformatted sources to standard output.
If a file's formatting is different from gofmt's, overwrite it
with gofmt's version.

Debugging support:

-cpuprofile filename
Write cpu profile to the specified file.

The rewrite rule specified with the -r flag must be a string of the form:

pattern -> replacement

Both pattern and replacement must be valid Go expressions.
In the pattern, single-character lowercase identifiers serve as
wildcards matching arbitrary sub-expressions; those expressions
will be substituted for the same identifiers in the replacement.

When gofmt reads from standard input, it accepts either a full Go program
or a program fragment. A program fragment must be a syntactically
valid declaration list, statement list, or expression. When formatting
such a fragment, gofmt preserves leading indentation as well as leading
and trailing spaces, so that individual sections of a Go program can be
formatted by piping them through gofmt.

Examples

To check files for unnecessary parentheses:

gofmt -r '(a) -> a' -l *.go

To remove the parentheses:

gofmt -r '(a) -> a' -w *.go

To convert the package tree from explicit slice upper bounds to implicit ones:

gofmt -r 'α[β:len(α)] -> α[β:]' -w $GOROOT/src

The simplify command

When invoked with -s gofmt will make the following source transformations where possible.

An array, slice, or map composite literal of the form:
[]T{T{}, T{}}
will be simplified to:
[]T{{}, {}}
A slice expression of the form:
s[a:len(s)]
will be simplified to:
s[a:]
A range of the form:
for x, _ = range v {...}
will be simplified to:
for x = range v {...}